Vacuum pump

ABSTRACT

A vacuum pump, particularly a turbomolecular pump or multi-inlet pump, comprises a rotor shaft ( 12 ) carrying at least one rotor means ( 14 ). The rotor shaft ( 12 ) is supported on the pressure side by a bearing assembly ( 56 ) and on the suction-side by a further bearing assembly ( 30 ). According to the invention, the suction-side bearing assembly ( 30 ) comprises an electromagnetic bearing ( 32,34 ). Further according to the invention, the pressure-side bearing assembly ( 56 ) comprises a ball bearing ( 58 ) so that the rotor shaft ( 12 ) can be given the largest possible length. The rolling bearing ( 58 ) is preferably biased by the electromagnetic bearing ( 32,34 ).

BACKGROUND

1. Field of the Invention

The present invention relates to a vacuum pump, particularly a turbomolecular pump or multi-inlet turbomolecular pump.

2. Description of the Prior Art

Turbomolecular pumps comprise a rotor means including at least one rotor having a plurality of rotor disks. Arranged between said rotor disks are stator disks held by stator rings. Said rotor means is arranged on a fast-rotating rotor shaft. Turbomolecular pumps have an inlet on the suction side and an outlet on the pressure side. At said suction-side inlet, final pressures of optionally less than 10⁻¹⁰ mbar can be generated. Said pressure-side pump outlet is often connected to additional pre-vacuum pumps.

Multi-inlet pumps comprise, in addition to a suction-side main inlet, at least one intermediate inlet. Normally, the rotor array of a multi-inlet pump comprises two pump stages which are formed e.g. as turbomolecular stages, said intermediate inlet being arranged between the two pump stages. Often, a further pump stage such as, e.g., a Holweck stage, is arranged downstream of the turbomolecular stages in the conveying direction. Multi-inlet pumps make it possible to generate different pressure levels at said main inlet and said at least one intermediate inlet.

Particularly in fast-rotating vacuum pumps such as, e.g., turbomolecular pumps and multi-inlet pumps, support of the rotor shaft on the pressure side, e.g. in regions where no low pressures prevail, can be provided via electromagnetic bearings. In known vacuum pumps, such electromagnetic bearings are used in pressure ranges up to 120 mbar. Further, it is known to support the rotor shaft in the high vacuum range by use of passive magnet bearings.

For support of a vacuum pump on the suction side, use of electromagnetic bearings is not a common practice, which is due to the low pressures prevailing in this region; notably, the coil bodies and sensor devices used in electromagnetic bearings are components with large surfaces and numerous hollow spaces. Consequently, because of the continuous outgassing, it is not possible or at least possible only with difficulties to reach the desired final pressure. Further, it is known to use permanent magnetic bearings in the high vacuum region.

For electromagnetic support of the whole rotor shaft, it is proposed in DE 20 2005 019 644 to arrange the two electromagnetic bearings within a cartridge. In said cartridge, the rotor shaft is arranged together with the bearings and the electric motor. The cartridge is open substantially in the direction of the pressure side so that, within the cartridge, there will prevail atmospheric pressure or at least a relatively high pressure acting onto the pressure side of the pump. The rotor shaft comprises an extension projecting out of the cartridge and carrying the rotor means. Thus, the rotor means is fastened on a cantilevered end of the shaft. Consequently, the constructional length of the pump is limited. Further, as a result of the attachment of the rotor means on said cantilevered shaft end, high forces will occur at the bearing sites, thus entailing the need to provide correspondingly complex electromagnetic bearings. Further still, this type of constructional design is subjected to massive restrictions due to the rotor-dynamic behavior since particularly low inherent frequencies will occur.

Further, e.g. from U.S. Pat. No. 6,416,290, it is known to provide a suction-side rotor on a cantilevered end of the rotor shaft. Thus, in the flow direction, the suction-side bearing is arranged behind the first rotor and consequently is not exposed to the very low pressures on the high vacuum side. Since, relative to the suction-side bearing, the attachment of the rotor is thus provided outside the bearing on a cantilevered end of the shaft, high forces and moments will occur in the bearings.

In turbomolecular pumps and particularly in multi-inlet pumps, it is desirable that the length of the rotor shaft is as large as possible so as to increase the overall pump performance and, optionally, to provide a larger number of intermediate inlets. With known bearing arrangements, the length of the rotor shaft is restricted because of machine-dynamic limitations. Presently, if the production and operating costs of a turbomolecular pump or a multi-inlet pump are to remain within a reasonable range, the maximum rotor length should be about 170 mm, with a rotor diameter of about 130 mm and a shaft diameter of about 8 mm.

It is an object of the invention to provide a vacuum pump, particularly a turbomolecular pump or a multi-inlet pump, wherein the rotor shaft has a larger length.

SUMMARY

In accordance with one aspect, a vacuum pump of the invention comprises a rotor shaft carrying a rotor means, said rotor means optionally comprising a plurality of rotors or other suction or pump devices. Said rotor shaft is normally supported via two bearing assemblies, notably a pressure-side bearing assembly and a suction-side bearing assembly. The suction-side bearing assembly is an electromagnetic bearing for radial support of the rotor shaft. Thus, with the aid of said electromagnetic bearing, there is effected exclusively a biaxial support of the rotor shaft. Herein, support in the axial direction is preferably not effected by the electromagnetic bearing. Further, the pressure-side bearing assembly is designed as a rolling bearing, particularly as a ball bearing. Use is made of such a rolling bearing, or the rolling bearing is arranged in such a manner, that the rolling bearing provides both axial and radial support, i.e. triaxial support of the shaft. The bearing assembly makes it possible to provide a long rotor shaft which will meet the high requirements to the quality and particularly also to the useful life of turbomolecular pumps or multi-inlet pumps. Particularly, such an arrangement has the advantage that a shaft which is supported on the shaft ends can be stiffened between the bearings as desired, without the necessity to give consideration to mating clearances for adequate bearings. The high stiffness of the shaft has the beneficial effect that the bending-critical inherent frequencies are shifted far away from the nominal frequency of the pump. Since bearings which are operated near the inherent frequencies of the shaft will be subjected to additional dynamic stresses, operation at frequencies far away from the inherent frequencies has a favorable influence on the useful life of the bearings.

The provision of an electromagnetic bearing on the suction side has the advantage that a contamination of the process gases will be avoided even at low pressures. If one were to use ball bearings with lubricants on the high vacuum side, the low pressures would cause an outgassing of the lubricant and thus lead to contamination of the process gases.

According to a particularly preferred embodiment, the suction-side bearing assembly serves exclusively for radial support. Thus, the rotor shaft is preferably not axially supported on the suction side. However, the electromagnetic bearing provided on the suction side preferably can also be used for biasing the rolling bearing located on the pressure side. This can be accomplished by a slight axial offset of the component parts of the electromagnetic bearing. Effecting the biasing of the rolling bearing with the aid of the suction-side electromagnetic bearing advantageously obviates the need for a separate biasing means so that no separate component need be provided. Instead, the biasing means is realized by the electromagnetic bearing.

Further, it is possible to realize the biasing of the rolling bearing with the aid of an electric motor. Preferably, said electric motor is arranged to act directly on the rotor shaft so that the rotor shaft will be directly driven by the electric motor. Thus, by a corresponding axial offset between the magnet and the coils of the electric motor, it is possible to generate a bias. For generating the bias by use of the electromagnetic bearing, this provision has the advantage that no additional component part will be necessitated.

Another possibility resides in the providing separate biasing means. Such a biasing means can be designed e.g. in the form of a magnet preferably arranged in the region of the rolling bearing. With preference, this magnet is a permanent magnet which thus will generate a clearly defined biasing force.

It is particularly preferred that the suction-side bearing assembly and/or the pressure-side bearing assembly are each arranged at a respective end of the rotor shaft. Thus, according to a preferred embodiment, the rotor shaft is not cantilevered on either of the two sides. Particularly in the region of the high vacuum, no cantilevered arrangement of the rotor shaft exists. This is possible due to the inventive arrangement and design of the bearings. In non-cantilevered shafts, merely relatively low forces and moments will occur in the bearings of the rotor shaft of a turbomolecular pump or a multi-inlet pump. Thereby, a rotor shaft of a smaller diameter can be provided particularly in the region of the electromagnetic bearing. Especially, this has the advantage that a relatively small catch bearing can be provided, thereby reducing the costs. Thus, in this preferred embodiment, all rotor assemblies are arranged between the bearings. At the most, the rotor assemblies may cover the bearings in the axial direction, but the rotor assemblies are always connected to the rotor shaft between the bearing assemblies.

Due to the inventive configuration of the bearing assemblies, it is rendered possible to realize rotor shafts having a length of more than 200 mm with a rotor diameter of about 130 mm, for use in turbomolecular pumps or multi-inlet pumps.

The electromagnetic bearing of the suction-side bearing assembly is preferably arranged within a cartridge. Thereby, it is made possible to arrange particularly the coil of the bearing in a region where the pressure is higher than the minimal pressure prevailing on the suction side.

The suction-side bearing assembly can be arranged in a high vacuum region and thus be subjected to low pressures. Further, the suction-side bearing assembly is an electromagnetic bearing. A high vacuum herein is understood to be a pressure of less than 10⁻³ mbar, preferably less than 10⁻⁵ mbar and most preferably less than 10⁻¹⁰ mbar.

Particularly if the electromagnetic bearing is arranged in the region of very low pressures as occurring on the suction side, i.e. in the inlet region of a turbomolecular pump, it is provided according to an especially preferred embodiment that the coil of the electromagnetic bearing is arranged within a pressure-encapsulated recess. By arranging the coil within a pressure-encapsulated recess, it is safeguarded that the coil itself is not located immediately in the high vacuum region. Thereby, the disadvantage is avoided that, due to the numerous hollow spaces within the coil, the resultant continuous outgassing would make it impossible or very difficult to reach the final pressure. By the inventive provision of an electromagnetic bearing in the high vacuum region, it is rendered possible to support the rotor shaft in the end regions of the shaft.

According to a particularly preferred embodiment, said recess is arranged in a housing element, i.e. preferably in a stationary element connected to the housing. An opening of the recess is preferably oriented in the direction of the rotor shaft. Particularly, the opening has a circular shape and fully surrounds the rotor shaft. Thus, an annular coil of a solenoid can be arranged in the recess. In such an arrangement, the electric feed lines preferably can be guided into the recess through the housing and not from the sides of the opening of the recess that is provided in the direction of the rotor shaft.

For pressure encapsulation, i.e. for sealing the recess, it would also be possible, for instance, to cast synthetic resin or the like into the recess after placing the coil in the recess. In case of very low pressures, however, the use of synthetic resin has the disadvantage of causing an outgassing of e.g. softening agents in the high vacuum, which in turn may adulterate the results of an analysis, for instance. According to a preferred embodiment of the invention, the opening of the recess is tightly closed by a preferably tubular closing element. The opening, preferably facing inwardly in the direction of the rotor shaft, can thus be closed in a simple manner by a tubular closure element. In case of a preferably circular recess, the opening of the recess corresponds to the inner peripheral surface of the circular cylinder. Preferably, the closure element can be sealingly connected to the housing element via sealing elements such as e.g. O-rings.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including the best mode thereof, enabling one of ordinary skill in the art to carry out the invention, is set forth in greater detail in the following description, including reference to the accompanying drawing in which the sole

FIG. 1 is a schematic sectional view of a multi-inlet pump comprising a bearing assembly according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A multi-inlet pump, as schematically illustrated in FIG. 1, comprises a rotor shaft 12 arranged in a housing 10. Said rotor shaft 12 carries a plurality of rotor means 14. Each of said rotor means 14 comprises a plurality of rotor disks 16. Between said rotor disks 16, stator disks 18 are arranged. A suction side 22 of the pump forms the high vacuum connection so that a medium will be sucked in the direction marked by arrow 24. Normally, an outlet 26 of the turbomolecular pump, i.e. the pressure side 28, is connected to a pre-vacuum pump.

The bearing assembly 30 arranged on the suction side comprises an electromagnetic bearing. This bearing comprises a coil 32 of a solenoid, and a bearing element 34 fixedly rotating together with rotor shaft 12, said bearing element being realized in the form of a so-called bearing plate. Said coil 32 is arranged in a recess 38 of a housing element 40. In the illustrated embodiment, said recess 38 has the shape of a circular cylinder. Recess 38 surrounds rotor shaft 12 in an end region of the shaft. Recess 38 is closed by a tubular closure element 42 and by sealing elements 44 preferably formed as O-rings. Thus, recess 38 is pressure-encapsulated. Consequently, within recess 38, there does not prevail the high vacuum existing in region 22. Thereby, it is avoided that the numerous hollow spaces located within the coil would make it difficult or even impossible to reach the final pressure.

Additionally, the illustrated suction-side bearing assembly 30 comprises a mechanical catch bearing 20 formed e.g. as a ball bearing. This catch bearing 20 is arranged in said housing element 40 and has a small distance to a shaft pin of shaft 12. Catch bearing 20 substantially serves for assuring emergency running features in case of failure of the electromagnetic bearing. In the illustrated embodiment, housing element 40 is pot-shaped and surrounds a suction-side end region of rotor shaft 12.

In the illustrated embodiment, the bearing assembly 56 arranged on the pressure side 28 is formed as a rolling bearing, particularly as a ball bearing 58. Said ball bearing 58 is designed and respectively is connected to a housing cover 60 in such a manner that the ball bearing 58 is able to take up both axial and radial forces.

Thus, the suction-side bearing assembly 30 serves exclusively for radial support of shaft 12, while the pressure-side bearing assembly 56 serves for axial and radial support of shaft 12.

With particular preference, it is provided that a radial bias is exerted on bearing 58. This bias can be generated with the aid of the suction-side bearing assembly 30 configured as an electromagnetic bearing. A corresponding bias can be generated already by a slight axial offset between coil 32 and bearing element 34. It is also possible to generate an axial bias of bearing assembly 56 with the aid of an electric motor 62. This motor 62, which in the illustrated embodiment is supported by said housing cover 60, comprises e.g. a permanent magnet 64 connected to rotor shaft 12, and a coil assembly 66. Also the axial offset between said permanent magnet 64 and said coil assembly 66 is suited to realize the bias of ball bearing 58.

In case of a multi-inlet pump, rotor shaft 12, being at both of its end regions supported by said bearing assemblies 30 and 56, comprises a plurality of condenser stages 76,78,80. In the illustrated embodiment, the first two condenser stages 76,78 are realized in the form of turbomolecular pumps, each them comprising a rotor 14 with rotor disks 16. Between said rotor disks 16, stator disks 18 are arranged. The two rotors 14 are arranged with a mutual distance on rotor shaft 12. Between the two rotors 14, housing 10 is provided with an inlet opening 82 which is the intermediate inlet.

The illustrated multi-inlet pump further comprises a main inlet 84 which is the high vacuum inlet. Via main inlet 84, the suctioned gas will flow in the direction marked by arrow 24. In the region of said intermediate inlet 82, an additional suctional intake of medium is performed via the intermediate inlet as indicated by arrow 86, and the suctioned medium will be conveyed towards the right-hand side in FIG. 1.

The third condenser stage 80 will then convey the medium, as marked by arrow 86, in the direction of the pressure side 28 and respectively the outlet 26. Normally, a pre-vacuum pump is connected to outlet 26. The third condenser stage 80 can comprise e.g. a Holweck stage or the like.

By the inventive configuration of the suction-side bearing assembly 30, it is rendered possible to provide the two bearing assemblies 56,30 at the shaft ends so that a maximum bearing spacing can be realized.

Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the true scope of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof. 

1. A vacuum pump, particularly a turbomolecular pump or multi-inlet turbomolecular pump, said vacuum pump comprising: a rotor shaft carrying at least one rotor means, and a pressure-side bearing assembly and a suction-side bearing assembly supporting said rotor shaft, said suction-side bearing assembly comprising an electromagnetic bearing which radially supports the rotor shaft, and said pressure-side bearing assembly comprising a rolling bearing which axially and radially supports said rotor shaft.
 2. The vacuum pump according to claim 1, wherein the suction-side bearing assembly serves exclusively for radial support.
 3. The vacuum pump according to claim 1, further including a biasing assembly which axially biases said rolling bearing.
 4. The vacuum pump according to claim 3, wherein said biasing assembly is formed by the suction-side bearing assembly.
 5. The vacuum pump according to claim 3, wherein said biasing assembly is formed by an electric motor which drives the rotor shaft.
 6. The vacuum pump according to claim 3, wherein said biasing assembly is formed by a separate magnet.
 7. The vacuum pump according to claim 1, wherein at least one of the suction-side bearing assembly and the pressure-side bearing assembly is arranged at an end of the rotor shaft.
 8. The vacuum pump according to claim 1, wherein all rotors are arranged between said bearing assemblies.
 9. The vacuum pump according to claim 1, wherein the rotor shaft between the two bearing assemblies has a length of at least 170 mm, with a rotor diameter of 100 to 140 mm.
 10. The vacuum pump according to claim 1, wherein the rotor shaft has a length between the two bearing assemblies that is larger than or equal to a diameter of the at least one rotor.
 11. The vacuum pump according to claim 1, wherein at least the suction side end of the rotor shaft has a reduced diameter.
 12. The vacuum pump according to claim 1, wherein the suction-side bearing assembly is arranged within a cartridge closed on the suction side.
 13. The vacuum pump according to claim 1, wherein the suction-side bearing assembly is arranged in a high vacuum region in which the pressure is less than 10⁻⁵ mbar.
 14. The vacuum pump according to claim 6, wherein the separate magnet is a permanent magnet arranged in the region of the rolling bearing.
 15. The vacuum pump according to claim 9, wherein the rotor shaft has a length of at least 220 mm.
 16. The vacuum pump according to claim 9, wherein the rotor diameter is about 130 mm.
 17. The vacuum pump according to claim 11, wherein the suction side end of the rotor shaft is smaller than or equal to 0.66 times a root of a rotor diameter of the at least one rotor.
 18. The vacuum pump according to claim 17, wherein the suction side end of the rotor shaft has a reduced diameter smaller than 6 mm.
 19. The vacuum pump according to claim 1, wherein the suction-side bearing assembly is arranged in a high vacuum region in which the pressure is less than 10⁻¹⁰ mbar.
 20. A turbomolecular vacuum pump comprising: a housing having a suction inlet adjacent a suction-side and an outlet adjacent a pressure-side; a rotor shaft; at least one rotor supported by the rotor shaft and at least one stator supported by the housing; an electromagnetic bearing assembly supporting a suction-end of the rotor shaft in an interior of the housing and adjacent a suction-side of the housing, the suction-side bearing assembly including: a magnetic bearing element affixed to and rotating with the rotor shaft, a coil mounted into the housing, and an isolation structure which vacuum isolates the coil from the interior of the housing; and a rolling bearing supporting a pressure-end of the rotor shaft in the interior of the housing adjacent the pressure-side of the housing. 